Helioseismic constraints on the structure of the present sun are examined using models with solar values of radius, luminosity, and mass, but with arbitrary profiles of the hydrogen abundance in the core. The hypothesis is assumed that within fixed physics (opacity, thermodynamics, and nuclear reactions) there is for all solar models a rigorous relation between the surface helium abundance and the entropy of the adiabatic part of the convective zone. This hypothesis is verified with models using somewhat simplified physics and with six evolutionary models computed by Christensen-Dalsgaard (1992). The relation depends chiefly on the opacity in the radiative zone between the lower boundary of convection and the region of nuclear reactions. The possibility of agreement between these relations and the helioseismic determination of the surface helium abundance is discussed.